Understanding the interactions between macrofauna organisms and sediment biogeochemistry and microbioloy is crucial in evaluating marine ecosystem functioning. Most of the seafloor is influenced by bioturbation, yet macrofaunal activity and its influences on biogeochemical processes are not well studied. The goal of the presented research was to combine innovative approaches and modern techniques to investigate the interactions of geochemical processes, microbial activities, and macrofauna assemblages in two marine habitats: coastal sediments and deep-sea methane seeps.

A coastal lagoon in Catalina Harbor, CA, was characterized by dense populations of the ghost shrimp Neotrypaea californiensis and the fiddler crab Uca crenulata. The shrimp lived permanently subsurface and maintained complex deep-reaching burrows that it ventilated constantly. The crab built shallow simple burrows, mainly for protection, and often left the burrow to forage. Differences in burrowing behavior were reflected in contrasting microbial diversities. Shrimp burrow microbial communities were similar to those found in surface sediments while crab burrow communities were not significantly different from subsurface sediment communities. Next, specific microbial activities were examined surrounding N. californiensis burrows. High levels of sulfate reduction (SR), along with nitrogen fixation, were found in and around the burrows, supporting the idea that bioturbation can lead to the formation of reduced microniches characterized by elevated microbial activities. 2-dimensional mapping of these microniches showed that sulfate reduction rates (SRR) in reduced microniches associated with burrows were 3 orders of magnitude higher than the surrounding sediment.

At western North American methane seeps, the seepage of methane-laden fluids supports sulfide-oxidizing microbial mats and rich communities of vesicomyid clams (Calyptogena) harboring sulfide-oxidizing symbionts. It is assumed that the sulfide supporting these communities is produced by the coupled reaction of SR and anaerobic oxidation of methane. Yet, it is relatively unknown what influence the clams have on benthic geochemical gradients and microbial processes. Three seep locations (Hydrate Ridge, Eel River, and Monterey Bay) were examined for the influence of clams on local biogeochemical processes. In all cases, the clams significantly increased SRR. Microsensor measurements indicated that the clams transported sulfate-rich overlying water deeper into the sediment where it could be used for microbial SR.